It is a very good book, exciting and with atmospheric description
of the scenes and the relationship of the children to each other,
but the escape plan is unfortunately totally illogical:

Why does none of the men at the boat remain and stays on guard while the other calls the ship?
They know nevertheless that two children still are on the island,
whose hiding place they did not find out. That is plentifully imprudent, isn't it?

If one presupposes the fact that all men go down to the dungeons, there results
another problem: Why in all world don't the children simply lock
the men in the dungeon and escape? The task, which Dick carries out down
in the passages, is very unnecessary and extremely dangerous.